The fermentation process begins as soon as the curling process ends. The term of “fermentation” was for the first time started in 1901, as the name of the oxidation process, started by tea ferments. Fermentation takes on the most important chemical changes, taking place in the process of tea making. These changes finally determine the taste of the future beverage.
To ensure the direct fermentation, the tea leaves are moved in the large, cool, damp and dark rooms, where they are laid out on a flat surface in layers, approximately four inches (10 cm) thick. The surface should have no reaction with the chemical components of tea, and the aluminum plates or specially treated wood are used most often. Ideal set of conditions for high-quality fermentation is the combination of maximum low temperature (around 150 C) with a maximum high humidity (around 90%).
Fermentation can take from 45 minutes to 3-5 hours. Time of the process depends on the combination of several factors: initial temperature of the tea leaf after curling, its humidity after withering, air humidity and ventilation characteristics of the room.
In the process of fermentation the leaves become darker (brown). Fermentation and withering of the “extracted” tea juice create the characteristic aromas of black tea - from flower tea to fruit, nut and spicy teas. The process of fermentation should be stopped at the moment, when the tea aroma reaches its optimal condition.